1. Field of the Invention
The present invention relates to a process for producing a flexible polyurethane foam. More particularly, the present invention relates to a process for producing a flexible polyurethane foam which employs as the blowing agent carbon dioxide, which is generated by the reaction of water with free isocyanate groups, and employs a specific diphenylmethane diisocyanates (hereinafter referred to as "MDI") and a specific polyphenylmethane polyisocyanates (hereinafter referred to as "polymeric MDI") as organic polyisocyanate components.
2. Related Background Art
Flexible polyurethane foams are used in a great quantity for cushioning materials for automobiles, furniture, and the like. As the isocyanate component for producing such flexible polyurethane foams, usually used is tolylene diisocyanate solely or a mixture of tolylene diisocyanate and polymeric MDI.
Tolylene diisocyanate and its analogues, however, involve problems in industrial hygiene because of its high vapor pressure, and further, its curing characteristics in foam production are not satisfactory. Moreover, as well known, the foamed products thereof will undergo large compression set particularly under wet and hot conditions, and is subjected to permanent set in fatigue, so that such type of foams are not suitable for uses in which foams may be exposed to moisture and heat, and may be compressed. Accordingly, the MDI type polyisocyanates have come to be used solely for the production of flexible polyurethane foams.
In production of flexible polyurethane foams, water is used as a carbon-dioxide-generating agent in combination with a chlorofluorocarbon as an auxiliary blowing agent to help the blowing action of carbon dioxide. The water generates carbon dioxide gas by the reaction of water with free isocyanate groups, and the carbon dioxide gas serves as the blowing agent.
Since the use of chlorofluorocarbons has come to be severely criticized all over the world, methods are now being investigated for producing a flexible polyurethane foam by employing carbon dioxide, which is generated by the reaction of water with free isocyanate groups, as the substantially sole blowing agent. The single use of an MDI type diisocyanates for producing a flexible polyurethane foam involves a disadvantage that a low density foam material is not readily producible without use of an auxiliary blowing agent in addition to water. For example, Japanese Patent Laid-open Application No. Sho-53-51299 discloses a process for producing a flexible polyurethane foam by the one-shot process by using a mixture of specific composition of MDI and polymeric MDI. In the use of such an isocyanate mixture, the desired foam density and a physical properties are not readily obtainable if trichlorofluoromethane is absent as the auxiliary blowing agent.